I was prompted to ask this question by a comment I made recently to an answer to a question about US politics. I said,

…a recent example of an apparently fixed political orientation
changing drastically in only a few years is the rise of the Scottish
National Party. In the 2010 UK general election the SNP won 6 out of
59 Scottish constituencies. In the 2015 UK general election the SNP
won 56 out of 59. Its rise in vote share wasn't as spectacular as the
number of seats won under the First Past the Post system suggests, but
was still a big change in only five years. The rise of the SNP in the
more complicated d'Hondt voting system used in the Scottish Parliament
was also dramatic

Talk of the death of Scottish Labour is premature. It still receives millions of votes. But it has fallen far. In the UK general election of 2010 it had 42% of the votes and won 41 out of the 59 Scottish constituencies. In the 2015 general election it had 24.3% of the votes and won just one single constituency. It was the first time since 1959 that Labour had not won the most seats in Scotland at a general election. In the Scottish Parliament election of 2016, Scottish Labour dropped to third place, being displaced by the Scottish Conservatives as the main opposition to the all-conquering SNP.

Why did this sea-change in political allegiances occur?

The answer cannot simply be that the people of Scotland liked how the SNP performed after it first formed a minority government in the Scottish Parliament after the Scottish election of 2007, though that is undoubtedly true. For the SNP their vote jumped sharply up between 2007 and 2011 but scarcely changed between 2011 and 2016. For Scottish Labour their vote held up moderately well between 2007 and 2011 then dropped drastically between 2011 and 2016.

Some of this is easy to explain. Thousands of "Yes" voters disappointed by the result of the independence referendum joined the SNP in the days immediately following 18 September 2014. Most of them must have come from Labour, although there will have been former Greens and other minor parties plus previous non-voters as well. (All accept that there is little exchange between the Conservatives and the SNP.) This movement related to actual party members rather than the less committed general public, but we can safely assume their political movement was in the same direction.

But the odd thing is that, as far as I can see the turning point in support for Labour occurred a whole year before the independence referendum, in September 2013. This graph from the site "What Scotland Thinks" of responses to polls asking Scottish voters how they would vote in a general election shows Labour's support holding steady for more than a year then suddenly starting to slide in September 2013. My intuitive feeling is that the temporary recovery in the three months before the referendum was prompted purely by the referendum, and that the underlying pattern was simply continuing the downwards trend started in 2013.

That sharp if brief peak from July-Sep 2014 does, however, suggest that the explanation for the fall of Labour popular among SNP supporters, namely that the Scots were prompted to abandon Labour by their disgust at seeing Labour leaders appearing alongside the hated Tories in the referendum campaign, is lacking. The time when Labour and Tory were together on the platform was also the time when Labour seemed to be recovering.

Do we have any hard information on what happened around September 2013 to cause Labour to fall and SNP to rise?

2 Answers
2

Polling companies have been heavily criticised recently for their methodology, and the site you mention has provided further information, though not explicitly.

The graph from whatscotlandthinks.org , from which your observations flow, provides background information in the linked 'Notes and methodology'. The question posed by the pollsters, actually or similarly, was, 'How would you be likely to vote in a UK General Election?' and posed only to voters in Scotland. It can be seen that the actual question varied considerably, while some polls, from time to time, collated those wishing to vote SNP with 'Others', and others only recorded data for those certain to vote.

Looking for events in a specific period to explain short-term fluctuations in polling may be a fools errand and may be due to the issues surrounding the reliability of the type of surveys ie whether internet, phone, or face-to-face, and other survey issues such as response/non-response bias.

The SNP sailed into power on a flagship policy of total independence for Scotland. The weight of this overarching proclamation was easily invoked against a particularly strong national identity such as Scots - and to a considerably enlarged sector of populace regardless of other policies. The Scottish people nearly sleepwalked/stumbled into total independence through SNP dogma and it still remains possible.

Simultaneously, the overall laughability of the Labour party was somehow subconsciously evident to Scottish people and as competition had notably declined.